Structural and functional insights into spermatogenesis and Sertoli cell plasticity in adult common carp (Cyprinus carpio)

Spermatogenesis in teleost fishes is organized into highly specialized cystic units, wherein Sertoli cells encapsulate and support cohorts of synchronously developing germ cells-a structural and functional arrangement that contrasts markedly with the non-cystic seminiferous epithelium of amniotes, in which Sertoli cells simultaneously interact with multiple germ cell stages. In this study, we conducted a comprehensive morphological and stereological analysis of spermatogenesis in the common carp (Cyprinus carpio), a species of considerable economic relevance to aquaculture. Our investigation delineated the progression of germ cell development across the proliferative, meiotic, and spermiogenic phases, identifying nine distinct spermatogonial generations. The Sertoli cell support capacity, reflected in a germ-to-Sertoli cell ratio of approximately 94:1, underscores the efficiency of somatic investment in this cystic system. Using a 5'-bromo-2'-deoxyuridine (BrdU) pulse-chase approach, we estimated the combined duration of meiotic and spermiogenic processes to be around seven days. Additionally, long-term BrdU-retaining cells (LRCs) were detected within both type An undifferentiated spermatogonia and Sertoli cell populations, indicating the presence of slow-cycling cells with extended label retention. Spatial analyses further suggested that some spermatogonia may localize near the intertubular compartment, consistent with previously described niche-like regions. Importantly, we identified a subpopulation of Sertoli cell-like nuclei that were not associated with germ cell cysts-herein referred to as "non-cyst-associated" or "free" Sertoli cells. Although their precise role remains to be clarified, their morphological features and retention of BrdU labeling suggest that they may participate in structural remodeling or cyst reassembly during the spermatogenic cycle. These observations contribute to an emerging view of somatic cell plasticity in the teleost testis. Taken together, our findings reinforce the conserved principles of cystic spermatogenesis in teleosts and provide new insights into the dynamic interactions between germ cells and their somatic environment.